Electrical insulating material



rial has been customarily used.

Patented Aug. 7, 1945 ELECTRICAL INSULATING MATERIAL Charles Gordon Harford, Wollaston, Mass., as-

signor, by mesne assignments, to Jefferson Electric Company, Bellwood, 11]., a corporation of Illinois No Drawing. Application February 1943 8617181 N0. 475,949

p 2 Claims. (01. zen-res) This invention relates to an improved electrical I insulatingmaterial which is particularly suitedfor insulation purposes in electric transformers and the-like.

- v In the assembly of the high tension coils of up mica sheets. These rubber-base-compositions and products are also much" less expensive than n are the usual built-up mica sheets.

transformers such as those used in-neon light 7 circuits, for example, it is customary to use built up sheets of mica. as insulators and shields. These sheets are formed or bent into suitable shapes and placed at appropriate positions in the transformers during assembly, e. g. between the coils and their supports, and between the coils and the frame of the transformer. After assembly, the transformers generally are filled with molten potting compound, usually asphaltic-base. which fills= the air spaces between the various parts and betwee'n them and the transformer casings.

The use of built-up mica sheets in this connection entails some difilculties, not only because of the increasing cost and scarcity of mica of proper size and quality, but also because of the necessity for building up sheets of proper size and characteristics from the small mica. flakes or tablets commonly available, of breakage or cracking on bending the sheets to form, and of delamination caused by entry of water into the The rubber base compound of this invention is composed principally of rubber and ground mica, together with conventional fillers and accelerators. They preferably contain at least as much mica as rubber, on a weight basis, and may even contain as much as about two parts by weight of mica to one of rubber. There should, however,-

be sufficient rubber to surround and bond the mica particles effectively and to serve asthe continuous phase of'the product. An-exampl of a suitable composition is as follows: I

Parts by weight Rubber (smoked sheets) 100 Sulfur 45' Stearic acid i 1 Paraflin 3 Zinc oxide 1 Du Pont 808 accelerator 2 Ground mica 150 edges of the sheets and by the heat of the pot- V ting compound. V It is an object of this invention to provide an insulating materialwhich may'be produced in sheet form or in other special shapes and utilized in electrical apparatus for purposes such as those for which sheet mica and other insulating mate Another object of this invention is toprovide an insulating material which is plastic during an intermediate stage of manufacture which enables insulators formed thereof to be preformed or bent to the desired shape prior to or after installation f in apparatus to be insulated and thereafter finally cured or hardened in situ.

vAn additional object of this invention is to provide insulating material which does not delaminate in use due-to heating or to the presence of moisture.

In accordance with the present invention, an

insulating rubber-base compound is provided hav ing mica particles distributed therein, from which composition insulators'of the desired shape and size can be formed and placed in insulating posi-v tion in a transformer or the like during assembly ties.

thereof. Insulators formed of such material show excellent dielectric strength. They are inherently free from any tendency to delaminate,

since they are not laminated products as are built fine as 325-mesh have been found satisfactory.

The mica preferably also is water-ground or otherwise washed with water to remove impuri- The mica component of this rubber-mica product is therefore much less expensive than that of built-up mica sheets, which require mica flakes or tablets of appreciable size.

The components other than rubber and mica in the foregoing composition may be varied in kind or proportions within the limits of good hardrubbermaking practice; r q

The foregoing materials are mixed homogeneously in internal mixers or on a rubber mill in accordance with conventional rubber procedure, after which the mix is preferably allowed to age for say twenty-four hours and may then be formed into desired insulator shapes or sheets of the desired thickness and vulcanized to give them the desired degree of hardnessor rigidity for use in the insulating art. If desired the aged sheets .or shapes may be given a partial cure only, pref- Where the partially cured insulators are thus placed in position, final curing thereof to their ultimate condition may be effected by the heat of the potting compound which is poured into the apparatus during the final stage of manufacture. Ordinarily, molten potting compound at a temperature of 400-450 F. is employed, and the heat of this compound generally is sufficient to farther cure the rubber-mica insulators to the desired hardness. However, in some instances this heat may be insufficient, due, for example, to there being a relatively large weight of comparatively cold metal parts as compared with the weight of hot potting compound. In many such cases it is common practice to use additional heat so as to maintain the filled transformer at such a temperature that the potting compound can fill all the voids before it finally sets, the compound being a solid at normal temperatures. Similar procedure may be followed to complete the-curing of the rubber-mica insulators when that is necessary.

Tests on representative samples of the rubbermica composition of this invention, after final cure, showed break-down voltages ranging between 25,000 and 30,000 volts for sheets having a thickness of 25 mils, which is equivalent to a dielectricstrength of at least 1000 volts per mil. This compares favorably with conventional mica sheets of mil thickness which have a dielectric strength of about 1000 volts per mil. The dielectric strength is reduced, however, if the mica sheets delaminate as frequently happens due to presence of moisture, or due to the heat of the potting compound. It is therefore evident that the rubber-mica composition of this invention is or particular value as an insulating medium in high-voltage transformers as well as other types of electrical apparatus.

While I disclose an embodiment of the invention for the purpose of illustration, alterations may be made therein without departing from the spirit of the invention.

I claim:

1. A vulcanizable composition containing one to two parts of mica for each part of rubber, and containing sulphur in a sufficient amount as to give a hard vulcanized rubber,

2. An article of manufacture comprising a sheet of vulcanized rubber composition consisting essentially of parts by weight of rubber,

45 parts of sulphur and parts of ground mica.

CHARLES GORDON HARFORD. 

